If you search for a leotard pattern online, there are a lot of options available for free; you could probably choose one of those and lengthen the torso of it (cut it in half at the waist, slide the bottom half down however many inches you want to lengthen it, adjust the waist to curve nicely between the two halves). I would suggest also finding a pattern for a knit turtleneck (either online or buying a pattern; the major pattern companies, Mcalls, Vogue, and Butterick, go on sale pretty often, and that make their stuff affordable). You can combine the two patterns to get what you're looking for without too much difficulty; trace the neck and sleeve openings from the turtleneck onto the leotard pattern (you can tape on extra paper if you need to, or just retrace the whole thing; I use wrapping paper when 'creating' patterns this way because I think it's easier to have one whole piece) and attach the turtleneck's sleeves and neck to the torso.
Any kind of body suit this form-fitting needs to be made with a knit--otherwise you won't be able to move in it. To get something that stretchy, it would be really good to find something with lycra or spandex in it, but artificial fibers like that don't breathe well; if you could find a cotton/lycra or cotton/spandex blend, that would probably be more comfortable. The fabric in the costume you posted looks like a lycra or lycra blend to me, though it's hard to say for sure from a picture. I know you want it to be relatively cool, but make sure the fabric is reasonably thick, or you'll run into unhappy things like panty lines.